This invention relates to a method of surface treatment of titanium metal.
Generally, titanium metal is heat-resistant and is substantially equal in strength to carbon steel. Also, it is known to have a good corrosion resistance because it forms an oxide film on its surface.
Also, as for pure titanium, it is possible to improve its workability and mechanical strength by forming an alloy with any metal, especially copper, tin, iron, aluminum, vanadium, chrome, cobalt, molybdenum, tungsten, etc.
As products made of a titanium metal such as pure titanium or titanium alloy, corrosion-resistant screws and bolts, spectacle frames, and medical and dental materials are known. In order to lower the friction coefficient and improve the wear resistance of such titanium metal products, plasma carburizing treatment is disclosed in Japanese patent publication 7-90542 filed by the inventors of the present application.
As disclosed in the publication, before carburizing treatment, it is necessary to remove an oxide film on the surface of the titanium metal. For this purpose, cleaning treatment is carried out in which deposits on the surface of the titanium metal are spattered by turning a mixture of hydrogen gas and argon gas into plasma.
Such cleaning treatment for a titanium metal is carried out at a high temperature of 700° C. or over in the same manner as in carburizing treatment. This is because at a temperature below 700° C., the surface would not be activated or infiltration of activated carbon would not be achieved sufficiently.
But in the carburizing and cleaning treatment by the conventional method at a high temperature of 700° C. or over, lowering of the strength due to softening of the metal matrix is unavoidable.
In particular, solution treatment is often carried out with a titanium alloy as a matrix and thereafter aging treatment at about 500-700° C. is carried out for precipitation hardening. But when it is subjected to heating treatment thereafter at 700° C. or over, a surface layer is formed which comprises a phase in which α-type (hexagonal system) and β-type (body-centered cubic system) structures are present in a mixed state. This causes not only the α type but β type structure to deposit on the surface of the titanium alloy, so that the effect of precipitation hardening of the α type by aging treatment lowers.
Also, if plasma carburizing is carried out on a titanium metal at a lower temperature than 700° C., carbon ions tend to become amorphous, so that they will not be carburized into the titanium metal but deposit on the surface in the form of soot or glass-like carbon.
An object of this invention is to carburize titanium metal so that the strength inherent to a titanium metal is maintained and excellent wear resistance and low friction coefficient and improved corrosion resistance are achieved.
Also, another object is to provide a carburizing treatment method in which when carburizing is carried out at a low temperature, carbon reliably infiltrates into between metallic atoms without turning to amorphous and depositing on the metal surface even at a lower temperature than 700° C.